The field of Isotope Geology investigates the isotopic composition of
major and trace elements contained in rocks and minerals, with the
aim to better understand geological processes. Isotopic techniques
are used to address a wide range of geological problems, such as
the age of the Earth, the origin and formation of magmatic rocks,
palaeotemperatures in sedimentary basins, palaeoclimatology, etc. Isotope
geochemistry forms an integral part of modern Earth Sciences and
numerous important discoveries have been made thanks to this research.
Awareness of these techniques is required to understand research
reports and geological interpretations based on isotopic methods.
Isotope geochemistry plays an important role in peripheral fields
of research such as planetology (origin and evolution of the Solar
system) and archaeology (origin and age of settlements, tools and
artifacts).
The use of naturally occurring radioactive isotopes to date minerals and
rocks is the oldest branch of isotope geology. The foundations of these
so-called isotopic or radiometric dating methods were laid shortly after the
turn of the XXth century with the discovery of the laws of radioactive decay
by eminent physicists such as Ernest Rutherford and Frederick Soddy
(Rutherford and Soddy, 1902a,b). The application of these principles to the
field of Geology and the calibration of the geological time scale were
pioneered by Arthur Holmes (1911, 1913, 1947). Initially, radiometric
geochronology was exclusively based on uranium and its daughter
products, but with the development of increasingly sensitive analytical
equipment, ever more isotopic ‘clocks’ were added over the course of
the century: Rb/Sr (Hahn et al., 1943), 14C (Libby, 1946), K/Ar
(Aldrich and Nier, 1948), 238U fission tracks (Price and Walker, 1963),
40Ar/39Ar (Merrihue and Turner, 1966), Sm/Nd (Lugmair, 1974),
etc.
During the 1960s, geochemists began to investigate the non-radiogenic
composition of igneous rocks with the aim to understand their source and
origin. This line of research greatly expanded over the course of the 1970s
and 80s and nowadays the isotopic composition of elements such as Sr and
Nd in rocks and minerals is an established petrogenetic indicator. The
discovery that the isotopes of the light elements (H, C, N, O, S) are
fractionated by physical and chemical processes dates back to the 1930s.
The isotopic composition of these elements can therefore be used to
detect and understand the hydrospheric and lithospheric processes
causing such fractionation (Urey, 1947). This has led to a better
understanding of the physiochemical conditions under which rocks
and minerals are formed. Temperature is the most important of
these parameters and the aforementioned elements are often used for
palaeothermometry.
These lecture notes cover the first half of an Isotope Geology module at UCL that deals with the geochronological aspects of the subject. The second part of the module deals with stable isotopes. It is taught by Dr. Philip Pogge Von Strandmann and covered in a separate set of notes. The core of the geochronology notes is formed by Prof. Peter van den Haute’s lecture notes (in Dutch) at the University of Ghent. This was expanded with additional material, notably on the subjects of cosmogenic nuclide geochronology (Section 8) and U-Th-He dating (Section 7.1). Some figures were modified from published sources, including Allègre (2008), Braun et al. (2006), and Galbraith (2005). These books are recommended further reading material, as is the detailed textbook by Dickin (2005), from which both Allègre (2008) and van den Haute heavily borrowed. Additional lecture material, including the data files used in the programming practicals of Section 12, can be found at http://github.com/pvermees/geotopes.